Orthogonal frequency-division multiplexing OFDM uses discrete multi-tone modulation. With OFDM, the tones are modulated on a large number of evenly spaced subcarriers using some m-ary of quadrature amplitude modulation (QAM) or phase shift keying (PSK). For example, OFDM allows only one transceiver on a channel at any given time. To accommodate multiple transceivers (nodes), an OFDM network must use time division multiple access (TDMA), or frequency division multiple access (FDMA).
Orthogonal frequency-division multiplexing access (OFDMA) allows multiple transceiver to concurrently access the same channel, where a channel includes a group of evenly spaced subcarriers. OFDMA distributes subcarriers among the transceiver so multiple transceiver can transmit and receive on the same single radio frequency (RF) channel using time division multiplexing (TDD), or different RF channel using frequency division multiplexing (FDD) on multiple subchannels. The subchannels are further partitioned into groups of narrowband “tones.” Typically, the number of tone in a subchannel is dependent on the total bandwidth of the subchannel.
A conventional star network includes one master node (master) and multiple slave nodes (slaves). The master communicates with all the slave nodes, and the slave nodes only communicate with the master nodes. Such networks can use OFDMA and TDD for channel access
In applications that require high reliability with relatively low latency, such as factory automation, data from all the slaves need to be retrieved within a latency constraint. The latency constraint is called a communication cycle.
The related application describes an OFDMA based medium access control mechanism and the corresponding frame structure design for these industry applications that have stringent latency and reliability requirements.
According to the related application, in case when transmission from a slave node fails, that particular slave should be given a retransmission opportunity to meet the required transmission reliability without violating the latency constraint. However, if the channel between the master and the particular slave is in deep fade, the retransmission may have high probability to fail again. To address this issue, a cooperative scheme can be used to further improve reliability and latency performance.